Wear resistance of a steel grinding rod generally improves with increasing hardness. However, attempts in recent years to further increase hardness to improve wear resistance have been unsuccessful because the increase in hardness has resulted in greater failure rates. The microstructure of a conventional heat treated grinding rod has a martensite surface and a pearlite core. The core may have occasional regions of bainite and martensite due to rod centerline segregation. Increasing the hardness of these pearlitic core rods has resulted in high levels of breakage during the cascading action of the rods in a grinding mill. Failure by breaking can be longitudinal or transverse. A longitudinal break normally starts at either end of a grinding rod and propagates along the longitudinal axis. A transverse break can start at any position along the length of the rod and propagates perpendicularly to the longitudinal axis. Rod failure in a grinding mill is unacceptable because of increased costs due to rod consumption and downtime to remove broken rods from inside the mill. Accordingly, steel manufactures optimize the depth and hardness of martensite formation into the rod cross-section without increasing the hardness of the core in order to prevent breakage.
U.S. Pat. No. 4,589,934 discloses a steel grinding rod having 0.6-1% carbon, 0.7-1% manganese, 0.1-0.4% silicon, 0.15-0.35% molybdenum, 0.2-0.4% chromium, the balance iron, all percentages being by weight. The outer surface of the rod has a martensitic microstructure having a hardness greater than HRC 50 and a pearlitic core having a hardness of HRC 30-45. To minimize breakage, it is proposed to have soft rod end portions having a hardness of HRC 35-50. After being heated to an austenitization temperature, end portions of the rod are not quenched when cooling the rod to prevent formation of a high hardness martensite microstructure thereon.
Nevertheless, a long felt need remains to improve wear resistance of a grinding rod by increasing the surface hardness. Increasing a rod surface hardness to HRC 55 and above while maintaining a rod core hardness of about HRC 40 continues to result in high breakage rates.